With refrigeration technology becoming increasingly complex, there is an increasing need for improved thermal expansion for controlling temperatures in compression/expansion refrigeration systems. The need is for both greater stability and more precise control.
Thermal electric expansion valves for regulating the flow of high pressure refrigerant employ a bimetallic strip responsive to temperature to control the position of a valve needle within a valve seat. The bimetallic strip changes shape with temperature, forcing a valve needle with greater or lesser force against a spring acting on the needle. Pressurized refrigerant flowing through the orfice defined between the valve needle and valve seat is therefore allowed to expand at a variable rate, determining the temperature of expanded refrigerant in a two phase state. A heater wire is wrapped around the bimetallic strip. A controller energizes the wire as determined by a circuit responsive to a temperature sensor placed in contact with the refrigerated load. The level set by the control circuit thus can offset the deformation of the strip, and position of the valve needle, so as to predetermine the level of temperature to be maintained in the system.
This type of thermal electric expansion valve is not fully dependable, stable or predictable, because the bimetallic strip is acted upon by other inputs in addition to the heater input. In addition, the bimetallic strip has a degree of hysteresis and is highly sensitive to temperature change. Additionally, this type of thermal expansion valve is costly and inefficient to operate as the system must be adjusted to properly control the valve.